Method of operating an ion source for a time of flight mass spectrometer



Jan. 3, i967 M. H. STUDIER E3,296,434

METHOD OF OPERATING AN ION SOURCE FOR A TI OF FLIGHT MASS SPECTROMETERFiled May 26, 1964 3 Sheets-Sheet 1 'F'- l ,Pulse DG. Pozaef :E@iff/@5501 ,y

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METHOD OF' OPERATING AN ION SOURCE FOR A TIME OF FLIGHT MASSSPECTROMETER Jan. 3, 1967 M. H. STUDIER 3,296,434

METHOD OF OPERATING AN ION SOURCE FOR A TIME OF FLIGHT MASS SPECTROMETERFiled May 25, 1954 3 Sheets-Sheet 5 x) M 5 am QB w u W Y \g\ x\\ wF/ w//LU C l l i o w n i? N s f. Ow A 'n w QM Q 1 L4M Ll I W www w n INVENTOR.,lyafizjz Sida/er United States Patent O 3,296,434 METHOD OF OPERATINGAN ION SOURCE FOR A TIME F FLIGHT MASS SPECTROMETER Martin H. Stndier,Downers Grove, Ill., assignor to the United States of America asrepresented by the United States Atomic Energy Commission Filed May 26,1964, Ser. No. 370,387 11 Claims. (Cl. 250-41.9)

The invention described herein was made in the course of, or under, acontract with the United States Atomic Energy Commission.

This invention relates to ion sources for use with time of flight massspectrometers and more specifically to an impuroved method of operationtherefor.

The conventional time of ight mass spectrometer comprises an ion source,a flight tube and a detector all housed in an evacuated structure.Operation of the ion source is critical for the proper function of theinstrument. The ion source utilizes an electron beam to produce ionswith the beam being generally operated in a pulsed mode. This mode,however, has the disadvantage of a low duty cycle with resulting lowsensitivity. Attempts have been made to operate in a continuous mode butto date, for a slight increase in sensitivity, poorer resolution andgreater noise exist therefor. Thus, the continuous mode is not generallyrecommended for use over the pulsed mode. (D. B. Harrington,Encyclopedia of Spectroscopy, Reinhold Corp., New York 1960, pp.628-647.) Further, no method of operation is presently available whereinsurface ionization sources may be used in a time of ight massspectrometer.

It is therefore one object of the present invention to provide animproved method of operating in a continuous mode an ion source of atime of flight spectrometer.

It is another object of the present invention to provide a method ofoperating an ion source of a time of flight spectrometer in a continuousmode with improved sensitivity, resolution and no increase in noise tosignal ratio over the conventional pulse mode operation.

It is still another object of the present invention -to provide a methodof operating an ion source of a time of flight mass spectrometer whereina surface ionization source may be used.

Other objects of the present invention will become more apparent as thedetailed description proceeds.

In general, the present invention comprises the steps of injecting a gasto be analyzed between the backing plate and the first grid of an ionsource in a time of flight mass spectrometer and passing a continuouselectron beam between the backing plate and the first grid to ionize thegas. A negative potential pulse is periodically applied to the firstgrid to attract ions created therefrom to the first grid. The negativepotential pulses are also integrated and applied to the backing plate tocreate a compensating ion attraction effect for the time duration of thetail end of the negative potential pulses. A negative potential ismaintained on the second grid of the spectrometer Where by ionsperiodically attracted to said first grid are accelerated. A solid maybe analyzed instead of a gas by making a filament thereof or depositinga sample on a filament and mounting the filament between the backingplate and the first grid of the spectrometer. No electron beam isrequired, ions -being produced by continuous electrical heating of thefilament. A slight continuous bias is applied to the filament. Theremaining steps therefor are the same as set forth for a gas.

Further understanding of the present invention will best be obtainedfrom consideration of the accompanying drawings wherein:

FIG. l is a schematic diagram illustrating the practice 3,296,434Patented Jan. 3, 1967 ICC Vof the present invention with a modified ionsource for a time of flight mass spectrometer.

FIG. 6 illustrates the practice of the present invention with a surfaceionization source for the apparatus of FIG. 5.

In FIG. l the backing plate 10 and two grids 12 and 14 are shownschematically for a conventional ion source of a time of flight massspectrometer. The gas to be ionized and analyzed is fed via an inlet 16to the space between backing plate 10 and first grid 12. An electronsource 18 generates an electron beam which is directed through theinjected gas to cause ionization thereof. In FIG. l, the electron beamis shown so that its direction is into the paper.

For the practice of `the present invention the electron ,beam isoperated continuously, thereby producing positive ions with a duty cycleapproaching It has been found that the electron ybeam operates toconfine the positive ions so formed within its boundaries. It isbelieved that the electron beam creates a potential well wherein theformed positive ions are confined.

A pulse generator 20 periodically generates a negative potential pulsewhich is applied to -the first grid 12. With lthe backing plateungrounded, this negative potential pulse destroys the potential well bydellecting the electron beam. The negative potential of the appliedpulse to grid 12 also acts to draw out the positive ions formed by theelectron beam. To prevent unfocused ions from being drawn out into theflight tube of the spectrometer lthe pulse applied to grid 12 isintegrated by resistor 22 and capacitor 23 applied to the backing plate10. Since the effect on the posiltive ions of a voltage applied to thebacking plate 10 is opposite to that of a voltage of the same polarityapplied to the first grid 12, the effect of the pulse on the backingplate 10 is an algebraic subtraction from the pulse applied Ito `thefirst grid 12. T'he wave shapes of the negative potential pulse appliedto grid 12 and the integrated pulse derived therefrom and applied to thebacking plate 10 are illustrated in FIGS. 2 and 3, respectively. Thealgebraic resultant of these two pulses is shown in FIG. 4

As may be readily seen from FIG. 2, the negative potential pulse appliedto first grid 12 has a long tail 26 thereon. The integration circuit 22is adjusted so that the waveform of the pulse applied to the backingplate 10 is such that the tail of the pulse applied to the first grid issubtracted and `the voltage gradient on the positive ions is actuallyreversed. Thus, the integrated pulse applied to the backing plate 10provides a compensating ion attraction effect for the time duration ofthe tail end of the negative potential pulse applied to the first grid12. This compensating ion attraction effect vastly reduces the noise andhence improves the resolution and sensitivity of the machine by bunchingthe formed positive ions. A high negative bias potential is maintainedon the second grid 14 of the ion source. This potential accelerates thepositive ions to a constant energy before they enter the drift tube ofthe spectrometer.

The above described method illustrates the basic method of the presentinvention as applied to a conventional two grid ion source. FIG. y5illustrates a modified ion source whereto the preferred method for thepresent invention V3 may be applied. In the conventional ion source forcontinuous operation a three grid ion source is used (D. B. Harrington,Encyclopedia of Spectroscopy, Reinhold Publishing Corp., New York 1960',pp. 628-647).

The embodiment of FIG. 5 adds a fourth grid to the conventionalstructure. Thus, the embodiment of FIG. 5 comprises a backing plate 28,and four grids 30, 32, 34 and 36. The backing plate 28 is groundedthrough a variable resistor 38. The first grid 30 is biased slightlypositive via D.C. voltage supply 40, potentiometer 42 and resistor 43.The second grid 32 is similarly positively biased from a D.C. voltagesupply 44 visa potentiometer 46 and resistor 47. A negative bias isapplied to third grid 34 from D.C. supply 48 via potentiometer 50 andresistor 51.

In operation, .a continuous electron beam is collimated and `directedinto a gas to be analyzed which is injected between the backing plate 28and the first grid 30. The electron beam, as hereinbefore described,acts to ionize the gas and confine positive ions resulting therefrom ina potential Well formed by the electron beam. The ions are drawn outfrom the well by the application of a negative pulse to the first grid.This negative pulse is shown in detail in FIG. 2. A variable resistor 52and capacitor 54 connected between the first grid and ground reduce thetail of the negative pulse by providing a low impedance return toground. Variable resistor 56 and capacitor 58 integrate the negativepulse and apply it to the backing plate 30 as hereinbefore described forFIG. 1. The backing plate pulse so derived subtracts the tail of thepulse applied to the first grid 30 and reverses the potential gradienton the positive ions. This prevents the drift of positive ions past thefirst grid 30 after the main bunch thereof has passed. The variableresistor 38 connected between backing plate 28 and ground provides aD.C. return thereby preventing the buildup of a charge thereon. Resistor38, to some degree, also shapes the pulse applied to the backing plate28. The positive bias on rst grid 30 provides a focusing action for thespectrometer through its effect on the location of the electron beam andthe ions produced therefrom.

The negative pulse applied to first grid 30 is also applied via acoupling capacitor 59 to second grid 32. This pulse combined with thepositive bias applied to grid 32 operates to make the grid 32 aneffective filter for stray ions that drift past first filter 30.

The third grid 34 as previously stated is continuously negativelybiased. This negative bias on the third grid 34 acts to minimize currentleakage between grids and also to impr-ove sensitivity. Similarly, thefourth grid is more negatively biased than the third grid, therebyproviding the final acceleration of the positive ions into the flighttube of the spectrometer.

Using a model 12 spectrometer havin-g a 100 cm. flight tube manufacturedby Bendix Aviation Corporation with a modified ion source as shown inFIG. 5, improved sensitivity and resolution resulted with no increase innoise using the following valued components.

Variable resistor 38 100K As described above, the potentiometers andresistors are adjusted so that the biasing and pulse shaping effectsthereof achieve maximum sensitivity and resolution with minimum noise.It is to be noted that the conventional spectrometer such as the model12, identified above, has adjustable permanent magnets mounted on theion source for collimating the electron beam. It has been found for thepurposes of the present invention that the adjustment of these magnetsis also critical to the sensitivity and resolution of the machine. Theyshould be adjusted to collimate the beam from which maximum ionizationis achieved within a minimum area. The model 12 spectrometer wasoperated satisfactorily at a pulse repetition rate of 10 kc. with 100,usec. between pulses applied to the first grid 30. The amplitude andduration of the applied pulses are shown to scale in FIGS. 2 and 3.

It is to -be understood that though the foregoing method is described asapplicable to the analysis of a gas injected into the area between thebacking plate and the first grid it is not to be so limited thereto. Themethod is equally applicable to the analysis of a solid.

Where the test sample is in the form of a solid, the present inventionmay be applied thereto by the use of surface ionization. FIG. 6 is aschematic of the backing plate 28 and first grid 30 of the apparatus ofFIG. 5 showing the mounting of a solid test sample 62 therebetween. Whenusing a solid as a test sample the solid should be deposited on or madeinto a filament 64 as shown in FIG. 6. This filament is spatiallymounted between grid 30 and backing plate 28. A second filament 66 isspatially mounted adjacent filament and directly above it as shown. A.C.supplies 68 and 70 are connected across filaments 64 and 66 via filamenttransformers 72 and 74 as shown. A positive D.C. bias is applied to eachof the filaments 64 and 66 from D.C. supplies 76 and 78 which areconnected to the center taps and 82 of the output windings 84 and` 86 oftransformers 72 and 74.

In operation, the A.-C. supply 68 is adjusted to provide a continuousheating current through filament 64 such that molecules of the solid 62under test are boiled therefrom. A.C. supply 70 is adjusted so that itprovides a continuous heating current to filament 66 to cause thefilament to be heated to a high temperature. Molecules boiled from solid62 strike the hot (up to 2500 C.) filament 66 which causes ionization ofsuch striking molecules.

A slight (up to 10 volts) continuous positive D.C. bias is applied tothe filament 66 and a slight (up to 10 volts) continuous negative D.C.bias is applied to the filament 64. It has been found that these biasescoupled with the bias on first grid 30 act to f-ocus ions formed by thesurface ionization. It is to be noted that the polarity and val-ue ofthe biases applied to filaments 64 and 66 is dependent upon theoperating temperatures of the filaments 64 and 66. Thus, if the filament64 becomes overheated, thereby emitting electrons which ionize gases inthe machine, the negative potential thereof will have to be reduced andpossibly be reversed to a positive bias. Further, as filament 66increases in temperature, the increase in temperature gives addedvelocity to ions formed therefrom which in turn causes defocusing. Tocorrect for this, the positive potential on the filament 66 is thereforereduced. The remainder of the steps are as set forth for the methodpreviously described for the apparatus in FIG. 5. It is to be noted thatthe utilization 0f the electron beam is negated, surface ionizationforming a replacement therefor. The electron beam, however, may beutilized in conjunction with the surface ionization whereby moleculesunionized by the heated filament 66 may be analyzed since the presenceof the filaments 64 and 66 does not interfere with the electron gun modeof operation.

Filament arrangements may be used other than the one shown in FIG. 6.Successful operation has been obtained by the filaments being mounted atright Iangles instead of parallel as shown. Further, a single filamenthas been used instead of a double filament as shown. The position of thefilaments between the backing plate 28 and first grid 30 is notcritical. The filaments may be .positioned anywhere between backingplate 28 and first grid '30, but it is preferred that they be excludedlfrom the area therebetween defined by the aperture of the first grid30.

The spacing -of the grids for any of the above methods is the same as inthe conventional machine (MW apart). Thus, in the modified embodiment ofFIGS. 5 and 6, the extra fourth grid 36 is similar to third grid 34 withthe usual spacing of 1A therebetween.

When surface ionization is used (a solid test sample) the ionizationefficiency may be higher than with the electron Ibeam continuous mode.However, the noise level thereof is higher than in the conventionalpulsed or present continuous electron beam mode. The resolution for thesurface ionization method is at least as good as for the conventionalpulsed electron beam mode but lower than the presently describedcontinuous electron beam mode. The preferred method for best sensitivityand good resolution for the surface ionization uses the filaments in themanner shown in FIG. 6 wherein they are parallel. This arrangement givesa better geometry for molecules hitting the hot filament 66 than wherethe filaments are at right angles 'with respect to each other.

Persons skilled in the art will, of course, readily adapt the teachingsof the invention to methods far different than the methods described.Accordingly, the scope of the protection afforded the invention shouldnot be limited t-o the methods illustrated and described above but shallbe determined only in accordance with the appended claims.

What is claimed is:

1. A method of operating a time of flight mass spectrometer ion sourcehaving a backing plate and first and second grids comprising the stepsgenerating positive ions between said backing plate and said first grid,periodically generating a negative potential pulse, applying each ofsaid negative potential pulses to said first grid, integrating each ofsaid negative potential pulses while applying said pulses to said firstgrid, applying each of said integrated pulses to said backing plate, andcontinuously `applying -a negative potential to said second grid.

2. The method according to claim 1 wherein said positive ion generationcomprises injecting a gas between said hacking plate and said firstgrid, and passing a continuous electron beam between said backing lplateand said first grid to ionize said gas.

3. The method according -to claim 1 wherein said positive ion generationcomprises forming the test sample into a filament, spatially mountingsaid test filament between said backing plate and said first grid,spatially mounting a second filament adjacent said test filament,passing a current through said test filament to cause heating thereofand the removal of molecules of said test sample therefrom, passing acurrent through said second filament to cause heating thereof and theionization of molecules of said test sample Vremoved from said testfilament, and applying bias potentials to said test and second filamentssuch that said ionized molecules of said test sample are repelled fromsaid second filament and attracted toward said test filament.

4. A method of operating a time of flight mass spectrometer ion sourcehaving a backing plate and first and second grids comprising the stepscontinuously generating positive ions between said backing plate andsaid first grid, periodically generating a negative potential pulsehaving a short rise time and long decay time, applying each of saidnegative pulses -to said first grid to attract generated ions thereto,integrating each of said negative potential pulses while applying saidpulses to said first grid, applying each of said integrated pulses tosaid backing plate to create a compensating ion attraction effect forthe decay time duration of each of said applied negative potentialpulses, and applying a continuous negative potential to said second gridto accelerate ions periodically attracted to said first grid.

5. A method of operating a time of flight mass spectrometer ion sourcehaving a backing plate and first, second, third and fourth gridscomprising -the steps generating positive ions between said backingplate and said first grid, periodically generating a negative potentialpulse, applying each of said negative pulses to said first grid,integrating each of said negative potential pulses while applying saidpulses to said first grid, applying each of said integrated pulses tosaid backing plate, continuously applying positive potentials to saidfirst and second grids, and continuously applying negative potentials tosaid third and fourth grids.

6. The method according to claim 5 Iwherein said negative pulse has anamplitude of approximately minus 260 volts.

7. The method according lto claim `6 wherein the negative potentialsapplied to said third and fourth grids have approximate values of minus1000 volts and minus 2.8 kilovolts respectively.

8. The method according to claim 7 wherein said positive ion generationcomprises injecting a gas between said backing plate and said firstgrid, generating an electron beam, and collimating said electron Ibeamto pass between said backing plate and said first grid to ionize saidgas.

9. The method according to claim 7 wherein said positive ion generationcomprises forming the test sample into a filament, spatially mountingsaid test filament between said backing plate and said first grid,spatially mounting a second filament adjacent said test filament,passing a current through said test filament to cause heating thereofand the removal of molecules of said test sample therefrom, passing acurrent through said second filament to cause heating thereof and theionization of molecules of said test sample removed from said testfilament, and applying bias potentials to said test and second filamentssuch that said ionized molecules of -said test sample are repelled fromsaid second filament and attracted toward said test filament.

10. The method according to claim 9 wherein the values of said biaspotentials applied to said test and second filaments do not exceedapproximately minus ten volts and approximately plus ten v-oltsrespectively.

11. A method of operating a Itime of flight mass spectrometer ion sourcehaving a backing plate and first, second, third and fourth gridscomprising the steps continuously generating positive ions between saidbacking plate and said first grid, periodically generating a nega-tivepotential pulse having a short n'se time and long decay time, applyingeach of said negative pulses to said first grid to attract generatedions thereto, integrating each of said negative potential pulses whileapplying said pulses to said first grid, applying each of saidintegrated pulses to said backing plate to create a compensating ionattraction effect for the decay time duration of each of said appliednegative potential pulses, continuously applying positive potentials tosaid first and second grids, continuously applying a negative potentialto said third grid, and continuously applying a potential to said fourthgrid having a value more negative than that applied to said third grid.

References Cited by the Examiner UNITED STATES PATENTS RALPH G. NILSON,Primary Examiner.

W. F. LINDQUIST, Assistant Examiner.

4. A METHOD OF OPERATING A TIME OF FLIGHT MASS SPECTROMETER ION SOURCEHAVING A BACKING PLATE AND FIRST AND SECOND GRIDS COMPRISING THE STEPSCONTINUOUSLY GENERATING POSITIVE IONS BETWEEN SAID BACKING PLATE ANDSAID FIRST GRID, PERIODICALLY GENERATING A NEGATIVE POTENTIAL PULSEHAVING A SHORT RISE TIME AND LONG DECAY TIME, APPLYING EACH OF SAIDNEGATIVE PULSES TO SAID FIRST GRID TO ATTRACT GENERATED IONS THERETO,INTEGRATING EACH OF SAID NEGATIVE POTENTIAL PULSES WHILE APPLYING SAIDPULSES TO SAID FIRST GRID, APPLYING EACH OF SAID INTEGRATED PULSES TOSAID BACKING PLATE TO CREATE A COMPENSATING ION ATTRACTION EFFECT FORTHE DECAY TIME DURATION OF EACH OF SAID APPLIED NEGATIVE POTENTIALPULSE, AND APPLYING A CONTINUOUS NEGATIVE POTENTIAL TO SAID SECOND GRIDTO ACCELERATE IONS PERIODICALLY ATTRACTED TO SAID FIRST GRID.